High-resolution imaging of extraterrestrial planetary surfaces

There have been significant improvements in recent years in high-resolution imaging of planetary surfaces beyond Earth. Three examples are described here: the narrow-angle camera (NAC) of the Imaging Science Subsystem (ISS) of Cassini at Saturn [1], the High Resolution Imaging Science Experiment (HiRISE) on Mars Reconnaissance Orbiter [2], and the NAC of the Lunar Reconnaissance Orbiter Camera (LROC) on LRO [3].

Each of these cameras was designed according to the constraints and science objectives of its mission. The ISS-NAC, built at the Jet Propulsion Laboratory, is optimized for imaging a diverse array of targets, typically at large ranges, and with high priorities given to multispectral imaging (via two 12-slot fi lter wheels)and movies with high time resolution. ISS images in framing mode (1024 × 1024 pixels, 0.35° field of view, 6 μrad/pixel). ISS was not optimized for the highest spatial resolution during the brief close flybys, but with spacecraft target motion compensation has been able to acquire a few unsmeared images at scales of only a few meters/pixel.

In contrast to Cassini, MRO and LRO are in tight circular orbits suitable for pushbroom imaging to optimize spatial resolution. MRO orbits at an altitude near 300 km, about as low as practical given the Mars atmosphere, whereas LRO orbits at near 50 km altitude. Hence, although the instantaneous fi elds of view (IFOV) differ by a factor of 10 (1 μrad/ pixel for HiRISE; 10 μrad/ pixel for LROC), the mapping scales are similar (0.3 m/pixel for HiRISE; 0.5 m/pixel for LROC). The FOVs are 1.14° for HiRISE and 2.85° for each of two LROC NACs, for a combined FOV of 5.58°. Maximum images sizes are 20,048 × 118,000 pixels for HiRISE and 5,000 × 52,200 pixels for each LROC-NAC. HiRISE, built at Ball Aerospace and Technology Corp., uses time-delay integration (TDI) with up to 128 lines to achieve a high signal:noise ratio, essential for high-quality imaging through the dusty atmosphere, but which places tight constraints on spacecraft pointing stability. Color imaging cannot be achieved on HiRISE or LROC via separate images through a movable filter wheel given the fast ground speeds and narrow FOVs; HiRISE acquires 3-color coverage over 20% of the swath width via additional CCDs with fi lter covers. There are two LROC NACs, built at Malin Space Science Systems, used to double the swath width, yet the combined mass (16.4 kg) is still signifi cantly less than that of ISS (30.5 kg) or HiRISE (65 kg).

Alfred S. McEwen is Professor in the Department of Planetary Sciences, University of Arizona, and Director of the Planetary Image Research Lab (PIRL). He has studied planetary surfaces for more than 25 years, including time at the U.S. Geological Survey prior to joining UA in 1996. Current research interests include volcanology, cratering, and remote sensing of planetary surfaces.